Structural and genetic studies of V-region genetic markers in mouse immunoglobulin light chains will be performed. Plasmacytomas will be induced in Balb/c-Ly-2a, Ly-3a congenic strains and myeloma proteins will be tested for the IB-peptide marker in mouse Vk-regions and for other L-chain polymorphisms. Cell fusion of normal spleen cells from IB-positive strains and drug resistant cultured myeloma cells will be performed, and heterokaryons expressing L-chain genetic markers will be identified. Proteins will be subjected to structural analysis to characterize the polymorphisms and will be used as immunogens to construct serological assays for L-chain polymorphisms. Detailed structural analyses of the Ly antigens on mouse thymocytes, T-cells, and lymphoma cells will be performed. The Ly-2 and Ly-3 alloantigens are of particular interest since they are determined by genetic loci closely linked to the IB-peptide marker, and they mark populations of peripheral T-cells which have killer and suppressor activity. Cell surface constituents of prothymocytes present in the murine foetal thymus at day 13 of gestation will be studied using radiochemical, immunofluorescence and cytotoxic assays. Rabbits will be immunized with prothymocytes present in the foetal mouse thymus at day 13 of gestation in order to identify surface antigens which may be specific for these cells. Such antigens may play a role in migration of these cells into the thymic rudiment, and the antisera produced will be used to trace back the lineage of these cells in embryogenesis. Similar immunizations will be performed with foetal and adult thymic epithelial cells in order to identify molecules peculiar to these cells which play a role in thymus development and T-cell maturation. Murine leukemia viruses will be introduced into organ cultures of foetal mouse thymus in order to study their infectivity and leukemogenicity in cells of the early foetal thymus.